1. Field of the Invention
This invention relates to a method for utilizing a sulfuric acid/peroxide liquid mixture. More particularly, it relates to a method for utilizing a sulfuric acid/peroxide liquid mixture that can be used with advantage as a method for re-utilization of a used-up sulfuric acid/peroxide liquid mixture.
2. Description of the Related Art
A sulfuric acid/peroxide liquid mixture is so far used in large quantities as a rinsing liquid for a semiconductor wafer mainly in semiconductor manufacturing processing in the form of a sulfuric acid/aqueous hydrogen peroxide or a sulfuric acid/ozone liquid mixture.
This sulfuric acid/peroxide liquid mixture is a representative rinsing solution used mainly for exfoliating a photoresist on a wafer or removing particles of metal, in particular heavy metal. This liquid mixture is used in significantly larger quantities than in the case of other rinsing solutions, such as ammonia/aqueous hydrogen peroxide, hydrochloric acid/aqueous hydrogen peroxide or dilute fluoric acid.
The result is that a large quantity of a waste liquid is generated after use. The waste solution of this liquid mixture presents the following problems in connection with disposal:    (i) If the waste liquid is mixed with water, significant heat evolution occurs (in connection with the heat of hydration of sulfuric acid);    (ii) peroxo acid (Caro's acid) exhibiting strong oxidating power is left; and    (iii) aqueous hydrogen oxide is left such that vigorous foaming (oxygen yielding) is likely to occur as a result of impacts or mixing of impurities.
Due to the above-described problems of the heat evolution, abrupt rise in pressure caused by foaming, or to possibility of deterioration of the equipment due to oxidation, it has been retained to be difficult, in view of safety or maintenance, to transport the waste solution directly to purify it to sulfuric acid. Thus, according to a customary process, the waste solution is diluted and cooled with a large quantity of water, neutralized with an alkali such as slaked lime, agglomerated with an inorganic flocculant and with a high molecular flocculant, in this order, and finally allowed to flow into an effluent as waste water.
Since the amount of water or chemical agents, such as flocculants, are used in an amount which ultimately is tens to hundreds of that of the original sulfuric acid/peroxide liquid mixture, the processing cost, that is the equipment or maintenance cost, cost of the chemical agents or of disposal of sludge, is increased. If this is to be deemed to be unavoidable, the amount of the waste material is also increased prohibitively.
It has so far been felt that the waste sulfuric acid/peroxide liquid mixture cannot be disposed of except by causing it to flow into an effluent, without the possibility of its re-utilization.